Piezoelectric device constructed of pentaborate crystal



May 22, 1951 JAFFE 2,554,332

PIEZOELECTRIC DEVICE CONSTRUCTED OF PENTABORATE CRYSTAL Filed Sept. 5,1949 +0 POTASSIUM PENTABORATE TETRAHYDRATE +Z RUBIDIUM PENTABORATETETRAHYDRATE THICKNESS- SHEAR MODE PLATE 4 INVENTOR.

IG. 5 HANS JAFFE ATTORNEY Patented May 22, 1951 PIEZOELECTRIC DEVICECONSTRUOTED OF PENTABORATE CRYSTAL Hans Jaffc, Shaker Heights, Ohio,assignor to The Brush Development Company, Cleveland, Ohio, acorporation of Ohio Application September 3, 1949, Serial No. 113,929

12 Claims. (01. 171-327) metry, cannot exhibit piezoelectricity.Concerning the remaining crystal classes, all that can be said ontheoretical grounds is that piezoelectric responses, as between electricfields having predetermined orientations with respect to thecrystallographic axes and mechanical strains of predetermined types, ofsuch crystals are not prohibited by the symmetry characteristics of thecrystalline material. However, it has not been found possible to predictthe magnitude of any such responses, and the magnitude may besubstantially zero or entirely negligible in any particular case, eventhough the substance under consideration belongs to one of the so-calledpiezoelectric crystal classes.

There are a number of crystal classes which are represented individuallyby one or more crystalline substances whose elastic, dielectric, andpiezoelectric properties have been investigated rather thoroughly.I-Iowevenno detailed or quantitative piezoelectric data heretofore wereavailable with regard to any crystal of the crystal class known as thehemimorphic or pyramidal class of the orthorhombic system, alsodesignated class C2v. Nevertheless, it has been determined that a platesuitably cut from a crystalline material of the class CZv may exhibit apiezoelectric response in regard to a thickness-controlled shear modeand that such a thickness-shear mode response is free of undesiredcoupling to other modes. Piezoelectric thickness-controlled shearcrystal plates of this type are described and claimed in the copendingapplication Serial No. 20,173, filed April 10, 1948, in the name of HansG, Baerwald and assigned to the same assignee as the present inventionwhich issued on October 18, 1949, as Patent No. 2,485,130.

A number of compounds were known to crystallographers to conform to thecrystalline structure of the hemimorphic orthorhombic class, class C2v.Notable among these substances is hemimorphite -(calamine), which,however, seems to be of no practical use in the piezoelectric art.

Small crystals of the substance known as potassium pentaboratetetrahydrate were investigated and the lattice parameters of thiscrystalline substance were measured as long ago as 1901. This workindicated that the substance did not belong to the hemimorphicorthorhombic crystal class. More recent work on this same compoundyielded revised information on its crystalline structure. The earlyinvestigations were found to be in error, and the substance was assignedproperly to the hemimorphic orthorhombic class. The more recent workalso indicated that the hydrogen atoms present in this substance aremore closely bound in the crystal lattice than is indicated by thetetrahydrate formula, and it was concluded that the true structure isrepresented more accurately by the formula potassium dihydrogendihydronium pentaborate. Even these detailed investigations, however,failed to give any indication that the substance under investigationexhibits a measurable piezoelectric response. In fact, it may beconcluded that, prior to the present invention, no substance was knownin the crystal class (2a which could be obtained in single-crystallineplates of a convenient size and chemical stability and which also wasknown to afford a piezoelectric response of any commercially significantmagnitude.

Accordingly, it is an object of this invention to provide apiezoelectric .device comprising a piezoelectrically sensitive sectioncut from a single crystal and having novel and useful piezoelectricproperties.

It is another object of the present invention to provide a new andimproved piezoelectric device comprising a section cut from asubstanceof the hemimorphic orthorhombic crystal class.

It isa further object of the invention to provide a new and improvedpiezoelectric device comprising a piezoelectrically sensitive sectioncut from a crystalline material which is stable to temperatures higherthan those usually encountered during the operation of such devices.

It is yet another object of the invention to provide a new .and improvedpiezoelectric device comprising a piezoelectrically sensitive resonatorsection having a desirably low frequency-temperature coeflicient.

It is a, still further object of the invention to provide a new andimproved piezoelectric device comprising a synthetic crystal platehaving a high piezoelectric response in a thickness-shear mode.

In accordance with the invention, a piezoelectric device comprises a.piezoelectrically sensitive section having electroded major surfacescut from a single crystal of a composition selected from the groupconsisting of potassium pentaborate tetrahydrate, rubidium pentaboratetetrahydrate, and isomorphic mixtures of these potassium and rubidiumpentaborate hydrates. While these pentaborate compositions are referredto herein as tetrahydrates, it will be understood that the compoundsintended to be designated are those commonly identified as tetrahydratesand given the formula MB5O8'4H2O, where M is the univalent metal,although some or all of the water of hydration apparently is bound moreclosely within the pentaborate molecule, so that the formula moreaccurately might be written MH2(H3O)2B5O10, an acid dihydroniumpentaborate. For convenience the designated compounds will be referredto in this specification and in the appended claims as pentaboratetetrahydrates. Furthermore, the term electroded surface as used in thisspecification and in the appended claims is intended to include anelectrode arranged so as to be spaced by an air gap from the crystalsurface and closely capacitively coupled thereto, as frequentlypracticed in the piezoelectric art.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawing, and itsscope will be pointed out in the appended claims.

In the drawing, Fig. 1 is a perspective view of a typical single crystalof potassium pentaborate tetrahydrate, or of an isomorphous compositioncontaining rubidium pentaborate tetrahydrate, which is useful in apiezoelectric device in accordance with the invention;

Fig. 2 is a representation in the same perspective as that of Fig. 1 ofa thin plate cut from the crystal of Fig. 1 with a predeterminedcrystallographic orientation and showing the outline within a portion ofthis plate of a piezoelectrically sensitive section having a rectangularshape which may be cut from the plate;

Fig. 3 is a plan view of the plate of Fig. 2 showing the samepiezoelectrically sensitive rectangular section;

Fig. 4 is a schematic circuit diagram representing an oscillator whichincludes a resonator section prepared from the rectangular sectionillustrated in Figs. 2 and 3; and

Fig. 5 is a view, partly schematic, of a transducer device comprising asimilar piezoelectrically i sensitive section.

Referring to Fig. 1, there is represented in perspective a singlecrystal of a composition which may be any composition selected from thegroup consisting of potassium pentaborate tetrahydrate, rubidiumpentaborate tetrahydrate, and isomorphic mixtures of the potassium andrubidium pentaborate hydrates, the latter type also being known as mixedcrystals. While any of the isomorphic compositions of this group mayhave 0 the external structure and development illustrated by way ofexample in Fig. 1, the crystal habit represented by Fig. 1 isparticularly common with potassium pentaborate tetrahydrate.

The pentaborate crystal is represented in Fig. 1 F

with relation to crystallographic axes a, b, and c. These threecrystallographic axes are made to coincide in the conventional mannerwith the respective coordinate axes X, Y, and Z of a righthanded systemof rectangular coordinates. positive directions of the crystallographicand coordinate axes are represented by arrows. The crystallographicindex of each of the crystal faces is designated in Fig. 1 in theconventional manner.

The

The piezoelectric phenomena discussed hereinbelow are described withreference to the coordinate axes X, Y, and Z. In order to clarify themanner of locating these coordinate axes in the crystalline material, adiscussion of some crystallographic characteristics of these pentaboratematerials is desirable. In accordance with accepted practice, thetwo-fold rotational axis present in class Czv is chosen as the c-axis,while the aand b-axes are perpendicular to the two symmetry planes.Axial ratios are determined by measurement of angles between theinclined crystal faces of index (111) and are found to bea:b:c=l.00:1.00:0.80. A distinction between the aand b-axes by means ofangular measurement is made difficult by the tendency of theaforementioned inclined faces to be curved.

The axes herein used, however, are readily identified on any crystal ofpotassium or rubidium pentaborate by the description of crystalproperties which follows: In terms of my axes, crystals of potassiumpentaborate and isomorphous compositions containing rubidium ordinarilyare characterized by the presence of a pair of faces perpendicular tothe b-axis, these faces having the crystallographic index (010). Thereis also a well developed face usually found at one end of the c-axis. Ihave chosen this end of the c-axis and the coinciding Z-axis as thepositive end; hence the well developed face referred to has the index(001+). I have found that an extensional stress applied parallel to theZ-axis produces a positive piezoelectric charge on the aforementionedpositive end of this axis.

These pentaborate crystals are further characterized by a pronouncedtendency to develop twinning during growth; the twinning plane is (101-)in terms of the crystallographic axes described hereinabove. The typicalcrystal illustrated in Fig. 1 has relatively small twinned portions ateach end of the crystal, and the twin boundaries are represented by wavylines.

On a piece of potassium pentaborate crystal devoid of well developedfaces, the crystallographic (1-, b-, and c-axes are readily identifiedby means of polarized light. The two optic axes are found to lie in theplane perpendicular to the a-axis, while the b-axis is the bisectrix ofthe acute angle between the optic axes.

The single crystals from Which the piezoelectrically sensitive sectionsof the device of the present invention are obtained show a pronouncedcleavage along YZ-planes, which also may be designated as (100) planes.Accordingly the major surfaces of an X-cut section or plate of thesesubstances, particularly of potassium pentaborate tetrahydrate, lie oncleavage planes, which greatly facilitates the production of X-cutsections of these materials. One such cleavage plane II is shown in Fig.1, passing more or less centrally through the crystal. This same plane Il is illustrated in the same perspective in Fig. 2, which also showsanother plane I2 which would be obtained by cutting through the crystalof Fig. 1 a short distance from the plane H and parallel thereto. Forsimplicity of illustration the plane I2 is not indicated in the view ofFig. 1. The crystalline material between the planes I and [2 has theshape of a thin plate l3 with two major surfaces normal to the directionof the X-axis and with edge portions which are portions of the originalfaces of the single crystal through which the cuts were passed along theplanes I! and 12. The crystallographic indices of these edge portionsalso are indicated in Fig. 2. In some cases 'deviations of the normal tothe plate I3 from the direction of the X-axis are permissible, andalteriinative directions .of the normal also are indicated in Fig. 2, asdiscussed hereinbelow.

Reference to Fig. 1 reveals at the top of the crystal a large (001)face, normal to the Z-axis. Although a small face parallel thereto anddesignated (001) appears at the bottom of the crystal, it should benoted that the latter face ordi- "narily does not develop during theunimpeded growth of a crystal of the type described. To

identify the (001*) face as artificial its outlines appear as dot-dashlines in the views of Figs.

'1-.3. The portions of the crystal of Fig. 1 which *ordinarily mightdevelop in the region beneath. the artificial (001-) face as shown inthe drawing would tend to be imperfect and twinned, and thus unfit foruse in the preparation of piezoelectrically sensitive sections.

Referring again to Fig. 2, there is shown within a portion of the plateI3 the outline of a section I4 having a rectangular shape which may beremoved from the plate I3 as by a sawing operation. The section I4 hastwo major surfaces in the planes I I and I2 respectively and is thenrelative to its length and width. The section is rectangular in outline,as seen more clearly in the plan view of Fig. 3, in which theX-direction is normal to the plane of the drawing. The longer edges ofthe section I4 extend in the Z-direction, while the shorter edges extendin the Y-direction. As mentioned hereinabove, however, in some cases thenormal may deviate somewhat from the X- 'direction, in which case atleast one of the edges no longer extends in the direction of acoordinate axis.

A suitable section, such as the section IA of Figs. 2 and 3, taken fromthe original single crystal advantageously may be incorporated as aresonator element in a frequency-selective piezoelectric device. X-cutsections are well adapted for resonator operation in the approximatefrequency range 0.4 me. to 5 me. While such a frequency-selective devicemay take the form of a crystal filter, in which these X-cut sectionshave the advantage of providing an unusually wide transmission bandwithdue to their high piezoelectric coupling coefiicient, it will beconvenient to describe a frequency-selective oscillator device. Thus thecrystal section be incorporated in an oscillator device of the typeillustrated schematically in Fig. 4. The crystal section I4 haselectroded major surfaces I6 and I1, illustrated schematically, theseelectroded surfaces being portions of the respective planes II and I2 asseen in Fig. 2. The electrodes or electroded surfaces I6 and I! may beprovided on the faces of the crystal section in any convenient manner.For example, various methods of applying thin coatings or layers ofmetal or other conductive material are well known. Alternatively, acrystal holder may be used which provides an air gap between theelectrode and the crystal surface. Thus, as described above, theresonator section l4 may be cut from a single crystal of a compositionselected from the group consisting of potassium pentaboratetetrahydrate, rubidium pentaborate tetrahydrate, and isomorphic mixturesof such potassium and rubidium pentaborate hydrates. Ordinarily it ispreferred to cut the section from a crystal-of potassium pentaboratetetrahydrate, since this substance is readily available and hasmentioned hereinbelow. However, sectio'ns'c'ut I 4 may from a crystal ofrubidium pentaborate tetrahydrate or from a mixed crystal of the twopentaborate hydrates may have somewhat diiferent properties which may bedesirable in certain cases.

The .remainder of the device of Fig. 4 comprises electrical oscillatorycircuit means for exciting the resonator section I4 so as to utilize thefrequency-selective characteristics thereof. This oscillatory circuit isa crystal-controlled circuit of a modified tuned-plate tuned-grid typehaving a triode vacuum tube I8. The tuned plate portion of the circuitincludes a variable capacitor as suitable for resonating a parallelinductor 2|. The parallel resonant circuit I9, 2| is connected acrossthe anode-cathode circuit of the triode I8 with a source of anodepotential 22 inserted between the resonant circuit and the groundedcathode of the triode. The essential element of the tuned grid portionof the circuit is the crystal resonator I4, the electrodes I6 and H ofwhich are connected to the cathode and control electrode respectively ofthe triode I8. The crystal element I4 is shunted by a resistor 23 and aseries choke inductor 24 to provide a suitable bias voltage. A capacitor2.6 may be connected between the anode .and control electrodes of thetriode I8, but this capacitor is shown in dotted lines because theinterelectrode capacitance of the triode ordinarily supplies the desiredcoupling between these electrodes.

The X-cut crystal section I l, having the shape illustrated in Figs. 2and 3, is adapted particularly well to be excited in a thickness-shearmode of motion so as to utilize the frequency-selective characteristicsof that mode of motion of the crystal section. In the operation of thecircuit of Fig. 4, capacitor I9 may be adjusted so that the circuit I9,2| resonates at a frequency which is at or near the frequency of athickness-shear resonance of the section I4. Any excitation in theanode-cathode circuit of .the triode I8 tends to produce oscillations atthe frequency of resonance of the tuned circuit I9, 2!. The resultingoscillatory voltage appears across the capacitance 26 and the impedanceof the crystal element I4, and stabilizes at such a frequency that it isapplied regeneratively to the control-electrode :circuit of the triode.This tends to set up oscillations, the frequency of which is determinedin a well-known manner by the steep impedancefrequency characteristic ofthe crystal element M The piezoelectrically sensitive crystal sectionincluded in the piezoelectric device of the present invention preferablyis cut from the pentaborate material and electroded so as to have a pairof electroded surfaces with the normal to the plane of each of thesesurfaces inclined not more than 15 from the X-axis of the crystallinesubstance.

When out in this way from a single crystal of the pentaborate,preferably of potassium pentaborate tetrahydrate, the crystal section ispiezoelectrically sensitive to a thickness-shear mode of motion. Thetrue X-cut section III illustrated specifically in Figs. 2 and 3, whichhas a pair of electroded major surfaces I6 and H as shown in Fig. 4 withthe normal to the plane of each of these surfaces substantiallycoinciding with the X-axis of the crystalline substance, has theadvantage that its major surfaces are out substantially along naturalcleavage planes of the original crystal. When an X-cut section, such asthe section IA, of potassium or rubidium. pentaborate tetrahydrate hasits thickness dimension,

parallel to the X-axis, much smaller than its length dimension, parallelto the Z-axis, the natural frequency of a thickness shear mode of.motion of the section is determined by the thickness dimension. Thesymmetry of this crystal line material is such that a field applied inthe 'X-direction produces a shear in the plane perpendicular to theY-axis, that is, the XZ-plane. The width dimension, parallel to theY-axis, may be chosen to provide a desirable capacitance for theelectroded section. It will be understood that operative piezoelectricdevices may include X-cut sections the edges of which are not parallelto the Y-directicn or Z-direction or which are not rectangular in shape.Modifications of the frequency constant or changes in thefrequencytemperature characteristics may be effected by cutting orgrinding the major surfaces of the crystal section so that the normal toeach surface is inclined somewhat from the X-axis of the crystallinesubstance, but, as indicated hereinabove, inclinations of more than 15from the X-axis may be expected to cause serious deterioration of thedesired piezoelectric response.

The orientation of such crystal plates deviating not more than 15 fromthe X-cut plates is indicated in Fig. 2 by the use of dashed linesrepresenting the direction of the normal to the plate relative to thedirection of the X-axis. It will be seen that the locus of the normalforms a cone with the axis of the cone in the X-direction. The anglebetween the normal and the X-axis may lie between 0 and 15, this anglebeing 0", of course, for a true X-cut section. It will be clear that theX-direction and one or both of the Y- and Z-directions no longer cancoincide with edges of the crystal plate when 0 is greater than zero,but for simplicity of illustration the edges are shown in Fig. 2 incoincidence with the directions of the coordinate axes, as is the casewith X-cut plates having rectangular major faces and the preferredorientation of the edges.

The thickness shear response of X-cut sections is not the only usefulpiezoelectric response obtainable with potassium or rubidium penta-.borate tetrahydrate crystals. The capabilities of,

for example, the potassium salt may be seen from the followingapproximate values of the piezoelectric coefficients in units of metersper volt:

dis 9.5 (124 1.7 C131 5.4-. daz Small (133 5.6

The coupling coeflicients are given by these piezoelectric coefiicientsin conjunction with dielectric and elastic data. The dielectricconstants for these materials are all about 5. A noteworthy feature ofthe elastic coefficients is the unusually high shear stiffness in theXZ-plane combined with unusually low compressional stiffness. Thedensity of the crystallized potassium salt is 1.74.

Three of the piezoelectric coefiicients given hereinabove havesufficiently high values to permit strong excitation of resonances.These are the coefficients dis, exciting the high frequency shear modearound the Y-axis in the X-cut section as described hereinabove, thecoefiicient 0131 providing a lateral expander mode in Z-cut sections,and the coefficient (133, which ordinarily might be expected to excitethickness expander modes of motion in Z-cut sections. Thickness,expander resonances of Z-cut sections have not been observed, however,presumably due to the opposite signs and almost equal value of thecoefficients (Z33 and (131 and strong elastic coupling between expansionin'the Z-direction and X-direction.

The X-cut thickness shear section I4 of Figs. 2 and 3 exhibits afrequency constant of about 1550 kc.-mm. at about 27 C., and eitherincrease or decrease of temperature causes the frequency constant todecrease, but only to the extent that the frequency is decreased by 0.1%at +2 C. and at +53 C. The coupling coefficient for the X-cut thicknessshear section is about 0.20. In the case of a thickness-shear mode, suchas discussed herein, the coupling coefficient is related to the ratio ofseries capacitance C5 to parallel capacitance Cp in the well knownequivalent circuit for piezoelectric crystal plates by the formula (coupling coefficient) (1r /8) (Cs/C11) In addition to the Z-cut lateralexpander mode mentioned hereinabove, useful piezoelectric responses maybe obtained in length expander and thickness modes with crystal sectionsor bars the thickness direction of which is inclined at an angle ofabout 45 to both the Z-axis and X-axis. Another bar with the thicknessdirection at about 45 to the Z- and Y-axes exhibits an expander moderesponse attributable to the (Z33 coefficient, but this response is notstrong. With a bar having its length direction inclined at 45 to the X-and Y-axes a lengthwise expander mode is obtained. A doubly inclinedsection, parallel to the (111) face, also acts as a piezoelectricresonator in several modes. Negative temperature coeflicients of thefrequency 'constant may be expected with most of these inclined cuts.Additionally, Y-cut plates may be excited in a thickness-shear mode byvirtue of the piezoelectric coefficient (124.

In addition to the perfect cleavage exhibited by the potassiumpentaborate tetrahydrate in Yz-planes, it may be remarked that thematerials show a good cleavage along XZ-planes. These materials have anadvantageously high stability at rather elevated temperatures; forexample, potassium pentaborate is stable to about C. It also may bementioned that the rubidium pentaborate tetrahydrate, the properties ofwhich closely resemble those of the potassium salt, has a maximumfrequency constant at about 5 C. Again, as with the potassium salt, anincrease or decrease in temperature of about 25 C. causes a decrease infrequency of about 0.1%.

Fig. 5 is a partially schematic representation of a piezoelectric devicefor transducing between the types of energy which are classified aselectrical and mechanical. Such a device may be used for transducingfrom electrical energy to mechanical energy, or vice versa. This devicealso utilizes a piezoelectrically sensitive section M cut from a singlecrystal of potassium or rubidium pentaborate hydrate. This section isseen in Fig. 5 mounted as if looking at the left hand edge of thesection M as shown in Fig. 2. In other words, the edge surface of thesection M as seen in Fig. 5 lies in an XZ-plane. The electroded surfaceIS of the section I4 is fastened securely to a supporting surface 30.The other electroded surface I! of the section [4 is fastened securelyto a solid rod 3|, only the left hand end portion of which is shown inthe drawing. A pair of terminals 32 is connected to the individualelectrodes [6 and H.

The arrangement of Fig. 5 may be used for transducing from electrical tomechanical energy by connecting to the terminals 32 the output circuitofan electrical signal generator, for example an ultrasonic frequencysignal generator. The resulting electrical field developed in thedirection of the X-axis of the crystalline substance causes sheardistortions of the XZ- planes within the section l4. Such a distortionat a given instant appears as a downward motion of the unmounted orright hand face of the section l4, as represented in Fig. by the arrows33. These shear motions alternate upward and downward as seen in Fig. 5and propagate to the right along the rod 3|. There also appear in Fig. 5several additional sets of arrows 34, 35, and 36, representing for thegiven instant the positions of regions of maximum deformation in upwardand downward directions due to motion of the element l4 previous to thatrepresented by the arrows 33. These additional sets of arrows, ofcourse, are spaced one half wave length apart along the rod, the wave.

length being determined by the velocity of propagation of ultrasonicenergy along the rod 3i. Thus the terminals 32 and the electricalcircuit apparatus connected thereto, together with the electrodes l6 andI1 and the interconnecting wires, constitute means for applying tomechanical energy is derived in the form of shear deformationspropagating along the rod 3! as described above, and the propagation ofthe ultrasonic energy along the rod 3| is a utilization of themechanical energy so derived. The ultimate utilization of the mechanicalenergy may be the testing of the rod 3| for flaws. The energy propagatedalong the rod 3| may be reflected from the distant right hand end of therod, not shown, and thus returned to the left hand end after the roundtrip time of propagation of the energy along the rod has elapsed. Forthis purpose the vibrational energy should be developed in the rod 3| inshort pulses of ultrasonic frequency energy, which can be generated in awell-known manner. If reflected pulses appear at the left end of the rodbefore this round trip period of time has elapsed, a structural flaw inthe rod is indicated. In another application of the Fig. 5 device therod 3|, of predetermined length and elastic properties, may be used as amechanical delay line for storing energy during the period of timerequired for round trip propagation of shear vibrations along the rod.It is noted that the mechanical energy transduced in the device isassociated with motion of the crystal section M in a thickness shearmode.

In the utilizations just discussed of the piezoelectric device of Fig.5, it was implied that shear vibrations propagating in the leftwarddirection along the rod 3| may be picked up by the element. In this casethe rod 3| and its mechanical connection to the mounted crystal sectionconstitute means for applying me-- chanical energy to the section,wherein this energy is transduced to electrical signal energy. Suitableelectrical receiving and display devices, well known in the art, may beconnected across the terminals 32, in which case such devices, the.

growing the isomorphous materials which may. be incorporated in thedevices of the present invention.

While there have been describedwhat are at present considered to be thepreferred embodi ments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,-aimed in the appended claims to cover all such" changes andmodifications as fall with the true spirit and scope of the invention.

What is claimed is:

1. A piezoelectric device comprising: a piezm electrically sensitivesection having electroded major surfaces cut from a single crystal of acomposition selected from the group consisting of potassium pentaboratetetrahydrate, rubidium pentaborate tetrahydrate,.and isomorphic mixtures of said potassium and rubidium pentaborate hydrates. Y 7 g 2. Apiezoelectric device comprising: a piezo electrically sensitive sectionhaving electroded major surfaces out from a single crystal of potassium' pentaborate tetrahydrate.

3. A piezoelectric device comprising: a piezo-t electrically sensitivesection having electroded major surfaces cut from a single crystal ofrubidium pentaborate tetrahydrate.

4. A piezoelectric device comprising: a section having electroded majorsurfaces, cut from a single crystal of a composition selected from thegroup consisting of potassium pentaborate tetrahydrate, rubidiumpentaborate tetrahydrate, and isomorphic mixtures of said potassium andrubidium pentaborate hydrates, and piezoelectrically sensitive to athickness-shear mode of motion of said crystal section.

5. A piezoelectric device comprising: a piezoelectrically sensitivesection, cut from a single crystal of a composition selected from thegroup consisting of potassium pentaborate tetrahydrate, rubidiumpentaborate tetrahydrate, and isomorphic mixtures of said potassium andrubidium pentaborate hydrates, and which has a pair of electrodedsurfaces with the normal to the plane of each of said surfaces inclinednot more than 15 from the X-axis of the crystalline substance.

6. A piezoelectric device comprising: a section, piezoelectricallysensitive to a thickness-shear mode of motion, cut from a single crystalof potassium pentaborate tetrahydrate, and having a pair of electrodedsurfaces with the normal to the plane of each of said surfaces inclinednot more than 15 from the X-axis of the crystalline substance.

'7. A piezoelectric device comprising: a piezoelectrically sensitivesection, cut from a single crystal of a composition selected from thegroup consisting of potassium pentaborate tetrahydrate, rubidiumpentaborate tetrahydrate, and isomorphic mixtures of said potassium andrubidium 11 pentaborate hydrates, and which has a pair of electrodedsurfaces with the normal to the plane of each of said surfacessubstantially coinciding with the X-axis of the crystalline substance.

8. A piezoelectric device comprising: a piezoelectrically sensitivesection, cut substantially along natural cleavage planes from a singlecrystal of a composition selected from the group consisting of potassiumpentaborate tetrahydrate, rubidium pentaborate tetrahydrate, andisomorphic mixtures of said potassium and rubidium pentaborate hydrates,and which has a pair of electroded major surfaces with the normal to theplane of each of said surfaces substantially coinciding with the X-axisof the crystalline substance.

9. A piezoelectric device comprising: a piezoelectrically sensitiveresonator section having electroded major surfaces cut from a singlecrystal of a composition selected from the group consisting of potassiumpentaborate tetrahydrate, rubidium pentaborate tetrahydrate, andisomorphic mixtures of said potassium and rubidium pentaborate hydrates;and electrical circuit means for exciting said resonator section so asto utilize the frequency-selective characteristics thereof.

10. A piezoelectric device comprising: a piezoelectrically sensitiveresonator section having electroded major surfaces cut from a singlecrystal of a composition selected from the group consisting of potassiumpentaborate tetrahydrate, rubidium pentaborate tetrahydrate, andisomorphic mixtures of said potassium and rubidium pentaborate hydrates;and electrical circuit means for exciting said resonator section in athickness-shear mode of motion so as to utilize the frequency-selectivecharacteristics of said mode of motion of said section.

11. A piezoelectric device for transducing between the types of energywhich are classified as electrical and mechanical comprising: apiezoelectrically sensitive section cut from a single crystal of acomposition selected from the group consisting of potassium pentaboratetetrahydrate, rubidium pentaborate tetrahydrate, and isomorphic mixturesof said potassium and rubidium pentaborate hydrates; means for applyingenergy of one of said types to said crystal section; and means dependentupon the effect of said applied energy upon said crystal section forderiving and utilizing energy of said other type.

12. A piezoelectric device for transducing between the types of energywhich are classified as electrical and mechanical comprising: apiezoelectrically sensitive section cut from a single crystal of acomposition selected from the group consisting of potassium pentaboratetetrahydrate, rubidium pentaborate tetrahydrate, and isomorphic mixturesof said potassium and rubidium pentaborate hydrates; means for applyingenergy of one of said types to said crystal section; and means dependentupon the effect of said applied energy upon said crystal section forderiving and utilizing energy of said other type; said mechanical energybeing associated with motion of said crystal section in athickness-shear mode.

HANS JAFFE.

REFERENCES CITED UNITED STATES PATENTS Name Date Mason June 2, 1942Number

